netdev - Re: eBPF - little-endian load instructions?

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Message-ID: <20170412165804.GA75807@ast-mbp.thefacebook.com>
Date:   Wed, 12 Apr 2017 09:58:06 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: eBPF - little-endian load instructions?

On Wed, Apr 12, 2017 at 03:02:00PM +0200, Johannes Berg wrote:
> On Tue, 2017-04-11 at 13:06 +0200, Daniel Borkmann wrote:
> > 
> > There are instructions to convert endianess, see __bpf_prog_run(),
> > the ALU_END_TO_BE, ALU_END_TO_LE labels for details. There's a
> > BPF_ENDIAN() macro used in the test suite and other places.
> 
> Are these hooked up to llvm intrinsics or so? If not, can I do that
> through some kind of inline asm statement?

llvm doesn't support bpf inline asm yet.

> In the samples, I only see people doing
> 
> #define _htonl __builtin_bswap32
> 
> but I'm not even completely convinced that's correct, since it assumes
> a little-endian host?

oh well, time to face the music.

In llvm backend I did:
// bswap16, bswap32, bswap64
class BSWAP<bits<32> SizeOp, string OpcodeStr, list<dag> Pattern>
...
  let op = 0xd;     // BPF_END
  let BPFSrc = 1;   // BPF_TO_BE (TODO: use BPF_TO_LE for big-endian target)
  let BPFClass = 4; // BPF_ALU

so __builtin_bswap32() is not a normal bswap. It's only doing bswap
if the compiled program running on little endian arch.
The plan was to fix it up for -march=bpfeb target (hence the comment above),
but it turned out that such __builtin_bswap32 matches perfectly to _htonl()
semantics, so I left it as-is even for -march=bpfeb.

On little endian:
ld_abs_W = *(u32*) + real bswap32
__builtin_bswap32() == bpf_to_be insn = real bswap32

On big endian:
ld_abs_W = *(u32*)
__builtin_bswap32() == bpf_to_be insn = nop

so in samples/bpf/*.c:
load_word() + _htonl()(__builtin_bswap32) has the same semantics
for both little and big endian archs, hence all networking sample code in
samples/bpf/*_kern.c works fine.

imo the whole thing is crazy ugly. llvm doesn't have 'htonl' equivalent builtin,
so builtin_bswap was the closest I could use to generate bpf_to_[bl]e insn.

To solve this properly I think we need two things:
. proper bswap32 insn in BPF
. extend llvm with bpf_to_be/le builtins
Both are not trivial amount of work.